1. Field of the Invention
The present invention relates to a field-effect transistor and a semiconductor device including it.
2. Description of the Related Art
A field-effect transistor (hereinafter, FET) used for a semiconductor integrated circuit or the like has been conventionally formed using a semiconductor such as silicon (see Non-Patent Document 1, for example). In recent years, a field-effect transistor including an oxide semiconductor having a band gap of 2.5 eV or more is reported. In particular, it has been revealed that by extremely lowering the donor concentration in a semiconductor layer, off-state current can be lowered to a value which cannot be measured by an ordinal method (see Patent Document 1, for example).
For example, the off-state current per micrometer of the channel width is generally larger than or equal to 1×10−15 A/μm in an FET including silicon, while the off-state current per micrometer of the channel width can be smaller than or equal to 1×10−18 A/μm in an FET including an indium-gallium-zinc-based oxide (In—Ga—Zn-based oxide) semiconductor. This is because the concentration of thermally excited carriers in an intrinsic semiconductor is extremely low due to its large band gap. When the band gap is greater than or equal to 3 eV, the smallest off-state current is smaller than or equal to 1×10−31 A/μm in theory.
When such an FET having extremely small off-state current is used in a dynamic random access memory including one FET and one capacitor (1T1C DRAM), the interval between refresh operations can be sufficiently longer. Ideally, data can be held semipermanently without a refresh operation (see Patent Document 2).
Further, when a gain cell memory including two transistors and one capacitor (see Patent Document 3) is formed using FETs each having an extremely small off-state current, a non-volatile memory which can hold data semipermanently can be provided. Although a gain cell memory, which has been proposed so far, does not need a capacitor with large capacity and has been regarded as an element to overcome disadvantages of the 1T1C DRAM, the off-state current of the gain cell memory cannot be sufficiently reduced in many cases, and thus, the gain cell memory has not been put into practical use.